I use Firefox on Android perhaps entirely because it supports uBlock Origin and my other extensions.
I would guess that of people that would ever go out of their way to use a non-Chrome browser on Android, the fraction who care about extensions is pretty significant.
On a different tack, I feel like I went out of my way to use Firefox (and Firefox Focus) on iOS and was thankful they had them during a time where everything had to use the safari renderer. IIRC Firefox Focus even had an ad-block extension that worked on safari
Firefox / Focus (like all browsers on ios) actually uses the "Safari renderer" (WebKit) because Apple doesn't allow any other browser engine on ios.
Historically yes, but in some areas like the EU there have been some regulation changes in 2024 where theoretically there could be alternate browser engines on iOS but in practice it hasn't happened yet. See https://open-web-advocacy.org/blog/apples-browser-engine-ban...
I would agree that it's probably significant. But it's probably not so high that a non-extensions-enabled Firefox for Android wouldn't be useful.